The present invention relates to a cold accumulating material for accumulating extremely low temperature cold used in refrigerators and the like and to a method of manufacturing this kind of cold accumulating material. More particularly, the present invention relates to an extremely low temperature cold accumulating material improved in mechanical strength and in chemical stability, having sufficient resistance to thermal shock and vibrations applied during operation and free from the risk of pulverizing into fine particles to make aeration of the refrigerant difficult, and relates to a method of manufacturing this cold accumulating material.
Recently, the superconduction technology has been remarkably developed for wide application to magnetic floating trains, stratigraphy diagnosis apparatus based on nuclear magnetic resonance, and the ultra high vacuum technology also has been developed for application to cryopumps for VLSI pattern transfer apparatus. With the coming of an era in which the super low temperature technology will be put into practical use, the development of smaller high-performance refrigerators for practical use is being promoted. In particular, the importance of refrigeration/cooling technology for providing an atmosphere at about the absolute zero degree (-273.degree. C.) in which superconducting electromagnets and high vacuum forming cryopumps for semiconductor manufacture apparatus can operate is increasing, and the provision of refrigerators improved in reliability as well as in characteristics is expected.
Conventionally, for superconducting MRI (nuclear magnetic resonance imaging) apparatus for taking straitigraphic photographs in medical fields, a Gifford-MacMahon type of small helium refrigerator (GM refrigerator), for example, is employed to cool a superconduction electromagnet by using liquid helium.
The GM refrigerator has a construction based on a combination of a compressor for compressing He gas, an expansion unit for expanding the compressed He gas and a cold accumulating unit for maintaining the cooled state of the He gas cooled in the expansion unit. The GM refrigerator effects cooling by expanding He gas compressed with a compressor in about 60 cycles per minute to cool the cooled system through an extreme end portion of the expansion unit.
Ordinarily, cold accumulating units of conventional refrigerators are constructed by packing at a high density a granular cold accumulating material mainly constituted by copper or lead as a main constituent or by packing multiple layers of meshlike cold accumlating members.
However, the volumetric specific heat of such a cold accumulating material or member formed of copper or lead abruptly decreases in the extremely low temperature range below 20 K. (-253.degree. C.), as shown in FIG. 8A. It is difficult to reduce the ultimate cooling temperature by using such a material. That is, where lead is used, the cold accumlating effect is lost in the temperature range below 10 K. (-263.degree. C.). Thus, the lowest temperature attainable with the conventional cold accumulating materials is considered to be 10 to 9 K.
The inventors of the present invention have eagerly studied to solve this problem, have developed a cold accumulating material having a large volumetric specific heat in an extremely low temperature range, and proposed this material in Japanese Patent Application No. 63-21218.
The cold accumulating material packed in the low temperature heat accumulator of this application is formed of a magnetic material which is a chemical compound constituted by a rare earth element and Ni, Co or Cu and having a large volumetric specific heat in an extremely low temperature range.
It was found that specifically erbium 3 nickel (Er-Ni.sub.1/3 also represented is Er.sub.3 Vi) has a volumetric specific heat generally equal to that of lead in the temperature range of ordinary temperature down to 15 K. (-258.degree. C.) but has a specific heat characteristics superior than that of lead in the extremely low temperature range below 15 K, as shown in FIG. 8A.
Ordinarily, conventional cold accumulating materials formed of such magnetic materials have been manufactured by a plasma spray gun apparatus such as that shown in FIG. 9.
This plasma spray gun apparatus 100 forms a plasma jet 105 of argon gas 104 by utilizing an arc discharge between an anode 102 and a cathode 103 and is supplied to a powdery raw material 106 which has been previously formed from ingot by mechanical pulverization so as to have a predetermined particle size. A surface portion or the whole of each particle of the supplied powdery raw material 106 is melted by heating it with the plasma and is simultaneously dispersed by the plasma jet 105. Each raw material particle is rapidly cooled and solidified while flying through a vacuum chamber 107 to be formed into the shape of a spherical particle 108.
This rounding enables the cold accumulating material to be packed in the cold accumulating unit at a large density.
Cold accumulating material particles prepared by the conventional plasma spray method, however, are essentially formed of a brittle intermetallic compound formed from a rare earth element and a metal such as nickel and have fine irregularities in their surfaces from which make the particles easy to crack. Moreover, micro-segregation occurs at grain boundaries and in grains. The strength of the particles is therefore small. The particles tend to be further pulverized by thermal shock, vibration, cooling gas flows and so on during refrigerator operation, and their effect is considerably disadvantageous. The cold accumulating material thereby reduced in size may clog in the cold accumulating unit and increase the resistance to the passage of He gas, which is the operating fluid. On the other hand, it may enter the compressor with the He gas to produce wear on the parts thereof.
Also, particles formed by this method are not uniform in shape and the particle size ranges very widely. Many of them have a large aspect ratio (ratio of major diameter to minor diameter), and the proportion of particles having a small size is particularly large. Actually, for cold accumlating particles packed into the cold accumulating unit, an additional classification step for removing excessively fine particles is required, which presents a problem in terms of economy. That is, the yield of the cold accumulating material relative to the raw material is very small, about 30%, and the efficiency at which expensive rare earth elements are utilized is small. Moreover, since the shape of the particles is not uniform, the density at which the cold accumulating material is packed in the cold accumulating unit is restricted and the cold storing efficiency is low.
According to the conventional plasma spray method, a cold accumulating material is formed from a raw material prepared by roughly pulverizing a cast alloy of a rare earth element and a metal by mechanical pulverization based on, for example, the stamping method into particles having a comparatively large size. There is therefore much segregation due to non-uniformity of the amount of melt inside and outside each particle, and the dispersion of the particle structure or composition depending upon the cast structure is large.
Specifically, in the plasma spray method, it is difficult to control the processing temperature, and the plasma generation temperature is extremely high. There is a possibility of some raw material components being evaporated at the variable and high processing temperature to further increase the non-uniformity of the structure. Accordingly, the possibility of formation of local electric cells in each particle is strong. Particle portions containing electric cells tend to oxidize and corrode faster. Thus, particles formed by this method are inferior in chemical stability.
The surfaces of cold accumulating material particles prepared by the plasma spray method are considerably rough and many irregularities and small cracks from which breaking of the particles may be started during use are formed in the surfaces, as shown in FIGS. 10A and 10B. It is considered that such irregularities and cracks reduce the mechanical strength and promote the reduction in the particle size.